In the interests of the environment, and to comply with increasingly stringent regulatory demands, it is necessary to increase the amount of biofuels used in automotive fuels.
Biofuels are combustible fuels, typically derived from biological sources, which result in a reduction in “well-to-wheels” (i.e. from source to combustion) greenhouse gas emissions. In diesel fuels for use in compression ignition engines, the most common biofuels are fatty acid alkyl esters (FAMEs), in particular fatty acid methyl esters (FAMEs) such as rapeseed methyl ester and palm oil methyl ester; these are used in blends with conventional diesel fuel components.
Due to the incomplete esterification of oils (triglycerides) during their manufacture, FAMEs can contain trace amounts of glycerides, in particular monoglycerides. These glycerides tend, on cooling, to crystallise out before the FAMEs themselves. This can compromise the cold weather operability of fuel formulations containing FAMEs, since the crystallised glycerides can block fuel filters.
The most common monoglycerides present in FAAEs are the saturated C16:0 (palmitic) and C18:0 (stearic) monoglycerides, and the unsaturated C18:1 (oleic) and C18:2 (linoleic) monoglycerides. The amount of each of these which is present in a FAAE will depend on the nature of the FAAE and also on the process by which it was manufactured. It is the saturated monoglycerides which appear to have the most detrimental effect on cold weather performance of FAAE-containing fuels, since they are less soluble than for instance triglycerides and more prone to precipitate at low temperatures; they are also typically present at higher levels than triglycerides (the European specification EN 14214:2003 for FAMEs for use as diesel fuels allows 0.8% w/w of monoglycerides but only 0.2% w/w of triglycerides). Certain monoglycerides are also thought to be responsible for corrosion and injector fouling issues in fuels containing FAAEs.
The addition of a FAAE to a diesel fuel formulation also raises its cloud point and cold filter plugging point (CFPP), to an extent dependent on the FAAE concentration. This too can compromise the cold weather performance of the resultant blend. It can therefore be difficult to formulate diesel fuel/FAAE blends within the relevant regulatory specifications, particularly in colder climates where specifications require maximum cloud points and CFPPs to be lower than in more temperate regions.
As a result, FAAEs are typically included in diesel fuels, in particular winter grade fuels, at relatively low concentrations. Moreover FAAEs for use in diesel fuels need to be prepared to relatively stringent specifications as regards their glyceride contents, thus increasing the cost of their production. FAMEs for use in current diesel fuels are typically required to contain a maximum of 0.8% w/w monoglycerides (EN 14214); a market survey conducted in July 2006 by the European Biodiesel Board (www.ebb-eu.org) showed that most marketed diesel-grade FAMEs contained only ˜0.44% w/w of monoglycerides.
It would be desirable to provide new biofuel-containing diesel fuel formulations which could overcome or at least mitigate these problems.